1. Field of the Invention
This invention relates to a scanning optical device and a method of regulating the imaging position in such a device. The scanning optical device of the invention can suitably be used for image forming apparatus which may be laser beam printers and digital copying machines.
2. Related Background Art
Scanning optical devices using a so-called overfilled optical system (OFS) are known as means effective for high speed and high resolution imaging. The overfilled optical system is characterized by the use of a rotary polygon mirror having a large number of reflection surfaces (deflection surfaces) and a light beam having a large width that irradiates any of the reflection surfaces. Since the reflection surfaces of the rotary polygon mirror are required only to have a substantial width necessary for deflection scanning in the wide light beam striking it, the rotary polygon mirror having a large number of reflection surfaces can reduce its diameter to make it suitable for high speed operation.
However, on the other hand, with the overfilled optical system, each of the reflection surfaces of the rotary polygon mirror moves in the wide light beam, changing it s angle relative to the latter as the polygon mirror revolves. Thus, if the width of the light beam is 1 when the optical axis is perpendicular to the reflection surface receiving it and the angle of rotation of the polygon mirror is xcex8, the reflected light beam shows a width D expressed by formula (1) below.
D=lxc2x7cos xcex8xe2x80x83xe2x80x83(1)
In other words, the spot of light formed on the surface to be scanned by the light beam incessantly changes its diameter in the main-scanning direction. Additionally, when a semiconductor laser is used for the light source of the scanning optical device, the light beam striking the rotary polygon mirror shows an intensity distribution referred to as Gaussian distribution. Thus, as the reflection surface moves in the light beam showing such an intensity distribution, the energy of the reflected (deflected) light beam changes as a function of the rotary motion of the reflection surface. Therefore, an image forming apparatus comprising an overfilled optical system is accompanied by the problem that the produced image shows an uneven density due to the change in the energy of the deflected light beam.
Since the overfilled optical system intrinsically has the above described characteristics, it is desirable that the light beam strikes the rotary polygon mirror along the optical axis of the scanning lens system (fxcex8 lens system) so that the scanning angle of rotary polygon mirror may swing symmetrically. With this arrangement, the light beam emitted from the light source strikes the rotary polygon mirror substantially along the center line of the deflection angle of the polygon mirror in the main-scanning section, i.e. a plane intersecting the optical axis along the main-scanning direction.
It is also desirable t hat the light beam strikes the rotary polygon mirror aslant in the sub-scanning section, i.e. a plane intersecting the optical axis along the sub-scanning direction, in order to avoid interference of the light beam striking the polygon mirror and the scanning light beam (deflected light beam) reflected by the polygon mirror. In other words, the light beam emitted from the light source desirably strikes the rotary polygon mirror with a predetermined angle and not perpendicularly relative to the deflection surface of the optical deflector in the sub-scanning section. Note that an angle of about 1 degree is selected f or the angle of incidence of the light beam striking the deflection surface in order to avoid any possible degradation of the image produced by the scanning optical device performing so-called deflection scanning so that the light beam striking the polygon mirror is completely isolated from the deflected light beam.
The scanning lens system of the scanning optical device is arranged near the rotary polygon mirror and the deflected light beam is focussed on the surface to be scanned and made to scan the surface at a constant scanning speed in the main-scanning direction. A lens having refractive power in the sub-scanning direction is arranged near the surface to be scanned to focus the deflected light beam in the sub-scanning direction. The incident light beam is also transmitted through the scanning optical system because of the positional arrangement of the latter so that the latter operates also as part of the optical system for incident light.
The optical system for incident light includes a condenser lens and a cylindrical lens for converging the light beam in the sub-scanning direction that are arranged between the light source and the scanning lens system. Thus, the light beam emitted from the light source is focussed to form a substantially linear image (extending in the main-scanning direction) near the reflection surface of the rotary polygon mirror by these lenses and the scanning lens system.
If the number of surfaces of the rotary polygon mirror is N, the scanning width of the light beam on the surface to be scanned is W and the scanning efficiency is xcex7, the focal length f of the scanning lens system is expressed by formula (2) below.
W=4xcfx80xcex7f/Nxe2x80x83xe2x80x83(2)
If N=12, W=352.2 (mm) and xcex7=0.9 are selected to fully exploit the characteristics of the overfilled optical system, f will be equal to 345 (mm) to realize a considerably long focal length for the scanning lens system. When the scanning lens system is made to have a long focal length, the position of the image on the surface to be scanned is significantly affected by the surface accuracy of the lenses of the lens system in terms of the displacement of the image so that the lenses have to be processed with an enhanced level of accuracy. When, on the other hand, the diameter of the spot of light formed on the surface to be scanned is reduced in the main-scanning direction to achieve a high degree of resolution, the depth of focus is reduced to by turn reduce the tolerance for the displacement of the imaging position of the scanning lens system. Any measures for alleviating these problems are costly.
Japanese Patent Application Laid-Open No. 10-206783 proposes a method of regulating the imaging position in the main-scanning direction of a scanning optical device by using a laser assembly where the position of the light source can be regulated by means of a holding member so that the operator can regulate the imaging position, while observing it on the surface to be scanned.
However, since the overfilled optical system shows a low efficiency of utilizing the quantity of light produced by the light source, the use of a semiconductor laser having a large output power exceeding 10 mW is normally required. Such a light source is apt to be damaged or otherwise degraded. This means that the light source should be regarded as consumable and therefore it should be arranged in the device as a component that can be replaced with ease. Otherwise, the entire output system should be elaborately regulated each time the light source is replaced to consequently raise the cost of running the device. In other words, the above known method of using a light source that can be positionally regulated is disadvantageous in terms of the use of a replaceable light source.
Another known method of regulating the imaging position of the overfilled optical system consists in unitizing the light source and the condenser lens and moving the unit along the optical axis.
However, to make this known method feasible, the unit has to be movable by several millimeters. Then, a mechanism that allows the unit to move by several millimeters is required to be subjected to a narrow tolerance for parallelism and eccentricity. Parts satisfying such a rigorous requirement are as a matter of fact costly.
In view of the above problems of the prior art, it is therefore the object of the present invention to provide a scanning optical device having an inexpensive configuration that allows to regulate the imaging position on the surface to be scanned easily and accurately and a method of regulating the imaging position of such a scanning optical device.
According to the invention, the above object is achieved by providing a scanning optical device comprising:
a light source;
an optical deflector having a deflection surface for deflecting the light beam emitted from the light source in the main-scanning direction;
a first optical system for causing the light beam emitted from the light source to strike the deflection surface of the optical deflector as a long linear image extending in the main-scanning direction; and
a second optical system for focussing the light beam deflected by the optical deflector on a surface to be scanned; the first optical system comprising a first lens, a second lens and a cylindrical lens showing refractive power in the sub-scanning direction perpendicular to the main-scanning direction, at least either the second lens or the cylindrical lens being movable along the optical axis to regulate the imaging position of the light beam on the surface to be scanned.
In another aspect of the present invention, there is provided an image forming apparatus comprising:
a scanning optical device having the above constitution;
a photosensitive member arranged at the surface to be scanned;
a developing unit for developing an electrostatic latent image formed on the surface of the photosensitive member by the light beam made to scan the surface by means of the scanning optical device into a toner image;
a transfer unit for transferring the developed toner image onto a toner image receiving member; and
a fixing unit for fixing the transferred toner image on the toner image receiving member.
In still another aspect of the present invention, there is also provided a method of regulating the imaging position on a surface to be scanned of an scanning optical device comprising a light source, an optical deflector having a deflection surface for deflecting the light beam emitted from the light source in the main-scanning direction, a first optical system for causing the light beam emitted from the light source to strike the deflection surface of the optical deflector as a long linear image extending in the main-scanning direction and a second optical system for focussing the light beam deflected by the optical deflector on the surface to be scanned, the first optical system comprising a first lens, a second lens and a cylindrical lens showing refractive power in the sub-scanning direction perpendicular to the main-scanning direction, the method comprising steps of:
regulating the imaging position of the light beam on the surface to be scanned in the main-scanning direction by moving the second lens along the optical axis; and
regulating the imaging position of the light beam on the surface to be scanned in the sub-scanning direction by moving the cylindrical lens along the optical axis.